This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. To understand the evolution of head form in tree-gouging marmosets, we are studying the jaw muscle position in the [unreadable] skull, basic anatomy and fiber architecture in marmosets and tamarins. Such insights into the basic biology of these [unreadable] small [unreadable] callitrichids will provide conservation biologists with information for making management decisions in critically [unreadable] endangered [unreadable] South American callitrichids.[unreadable] [unreadable] We are investigating and comparing the jaw-muscles of three South American monkey species. We are examining jaw-[unreadable] muscle morphology and position as well as muscle structure, fiber length and physiological cross-sectional area. The [unreadable] three [unreadable] species are common marmosets (Callithrix jacchus), saddle-back tamarins (Saguinus fuscicollis), and squirrel monkeys [unreadable] (Saimiri sciureus). C. jacchus habitually gouges trees for food, that is, it elicits gums from trees by mechanically [unreadable] damaging [unreadable] them with its anterior teeth. This tree gouging is hypothesized to require relatively large gapes. The two comparative [unreadable] taxa, [unreadable] S. fuscicollis and S. sciureus, both feed on insects, fruits and gums, but do not gouge trees. We are testing whether the [unreadable] jaw muscles of C. jacchus are positioned in a way that allows larger gapes (i.e., reduces the amount of stretching on [unreadable] them [unreadable] during jaw opening) as compared to S. fuscicollis and S. sciureus. We also are looking at whether C. jacchus has [unreadable] relatively [unreadable] longer muscle fiber lengths to facilitate these larger gapes, as compared to S. fuscicollis and S. sciureus. Finally, we are [unreadable] determining if these three species are able to produce comparable jaw forces across a range of gapes using data on [unreadable] physiological cross- sectional area. This work complements comparative functional analyses of skull form in these species [unreadable] and on-going field and lab work on tree gouging in common marmosets. This research used WNPRC Animal Services and [unreadable] Pathology Services.[unreadable] [unreadable] Publication (No PubMed ID#): Vinyard C.J., and T.M. Ryan. Cross-sectional bone distribution in the mandibles of [unreadable] gouging and non-gouging platyrrhines. Int. J. Primatol. 27:1461-1490. 2006.[unreadable]